Gas turbine engines are used to power aircraft, watercraft, power generators, and the like. Gas turbine engines typically include one or more compressors, a combustor, and one or more turbines. In typical aerospace applications, a fan or propeller is used to draw air into the engine and feeds the drawn-in air to the gas turbine core. The compressor includes alternating stages of rotating blades and static vanes, which increase the pressure of the drawn-in air as it travels through the gas turbine core. The compressor thus outputs higher-pressure air, which it delivers to the combustor. A fuel pump supplies pressurized fuel (such as kerosene) to the combustor, typically via one or more fuel injectors. In the combustor, the fuel is mixed with the higher-pressure air and is ignited by an igniter. The products of the combustion reaction that occurs in the combustor (e.g., hot gas) are directed into a turbine. The turbine is typically made up of an assembly of discs with blades, which are attached to turbine shafts, nozzle guide vanes, casings, and other structures. The turbine converts the thermal energy supplied by the combustion products into kinetic energy. The work extracted from the combustion products by the turbine may be used to drive the fan, the compressor, and, sometimes, an output shaft. Leftover products of the combustion are exhausted out of the engine and may provide thrust in some configurations.
Aerospace applications of gas turbine engines include turboshaft, turboprop, and turbofan engines. In typical aerospace applications, the gas turbine engine provides thrust to propel the aircraft, and also supplies power for engine accessories and aircraft accessories. Typical engine accessories include an engine control unit, a starter, fuel pumps, oil pumps, etc. Typical aircraft accessories include hydraulic pumps and electric generators to supply hydraulic and/or electrical power to the aircraft cabin and/or aircraft electrical systems.
The gas turbine engine provides thrust via the main engine shafting, which is driven by the turbine as a result of the combustion reaction. The gas turbine engine also powers engine accessories and aircraft accessories by one or more accessory drives, as further explained below. Some gas turbine engines also output bleed air, which is compressed air produced by the compressor but not delivered to the combustor. Bleed air can be used by the engine to, for example, generate propulsion thrust, drive a pneumatic actuator or provide engine cooling, or may be supplied to the aircraft for cabin pressurization or other purposes.
Mechanical power is transferred from turbines to compressors through shaft and spline systems, with bearings providing axial and radial positioning of the rotating components. A central shaft (which may be referred to as a “main” shaft, a “main drive,” or a “mainline” shaft, for example) typically links the turbine and compressor sections of the turbine engine. In turbine engines having multiple turbine and compressor sections, there may be multiple, concentric, independently rotatable shafts. For example, a high pressure shaft may link a high pressure compressor with a high pressure turbine, while a low pressure shaft links the fan with a low pressure turbine. The low pressure shaft may be concentric with and disposed within the high pressure shaft. In order to extract power from the turbine engine to drive the engine and aircraft accessories, one or more mechanical or electrical “accessory” drives transmit power from the engine mainline shafts to the engine and aircraft accessories. For example, a bevel gear, alone or in combination with other components (such as a stub shaft, idler, spur gears and splines), may be driven by a mainline shaft. The bevel gear may drive an accessory gearbox, which in turn runs the engine accessories and/or aircraft accessories. During engine start-up, an accessory drive can be used “in reverse” to transmit power from a starter to the engine.
Classic “power take-off” systems include mechanical drives that transmit power from the gears mounted on the engine mainline shafts to an accessory gearbox to drive accessories such as pumps and generators. However, the conventional accessory gearbox can be replaced by “more electric” systems that have embedded electrical starter/motor/generators mounted directly on the engine mainline shafts. Power electronics can convert the variable frequency electrical power that the engine delivers into alternating current (AC) or direct current (DC) as need by the engine and aircraft accessories.